1. Field of the Invention
The present invention relates generally to a device manufacturing method using lithography.
2. Description of the Prior Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
An illumination system is provided for receiving a beam of radiation from a radiation source and for supplying a conditioned beam of radiation, referred to as the projection beam, having a desired uniformity and intensity distribution in its cross-section for illuminating a reticle and for patterning with the reticle. The source and the lithographic apparatus may be separate entities, for example when the source is a plasma discharge source. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is generally passed from the source to the illumination system with the aid of a radiation collector comprising, for example, suitable collecting mirrors and/or a spectral purity filter. In other cases the source may be an integral part of the apparatus, for example when the source is a mercury lamp.
The resolution with which a mask pattern can be replicated into a layer of resist is dependent upon a number of factors. Chief amongst these factors is the wavelength of the illuminating radiation. Diffraction occurring at the mask will tend to reduce the resolution of the illuminating pattern. This reduction in resolution will be less for radiation of relatively short wavelength. Much research has therefore gone into producing systems which operate at lower and lower wavelengths. A present goal is to provide systems which operate in the so-called “extreme” UV range, that is at wavelengths of less than 50 nm, e.g., 13.4 nm or 11 nm. It is noted that EUV radiation is readily absorbed by resist material and therefore processing making use of EUV must operate with extremely thin resist layers, typically on the order of 100 nm.
As the wavelength of the radiation gets smaller, so the energy of photons impinging on the resist increase giving rise to an increase in the production, within the resist, of secondary electrons. Indeed, as at EUV wavelengths the photon energy no longer matches the binding energy between resist molecules, it is the secondary electrons which provide the main mechanism for exposing the resist. Some background information is helpful in understanding this phenomenon and also why secondary electrons can reduce the mask transfer resolution.
During exposure of a resist layer, photons impinging on the resist are absorbed by electrons bound within atoms of the resist, imparting sufficient energy to these electrons to allow them to escape from their respective atomic shells. Vacancies arise in certain of the atomic shells of the atoms. This process is known as “photoionisation”, and the freed electrons are referred to as “photoelectrons.” Important parameters in defining the extent of the region exposed are:                the mean free path of photons, determined by absorbance in the resist;        the energy levels at the different shells of the atoms;        the atom density of the resist; and        the angle distribution of the emitted photoelectrons.        
When an electron moves to fill the vacancy left by a photoelectron, a photon will be emitted with energy defined by the valence band difference between the old and the new state. This process is a form of fluorescent emission. Hence, further parameters defining the extent of the region exposed are:                available transitions (energy differences) between shells;        the probability that transfer will occur; and        the angle distribution of the fluorescent emission.        
When a photoelectron, collides with a bound electron, the impulse may be strong enough to knock out the bound electron to provide a “secondary” electron. The electron will travel in a new direction with reduced energy. Consequently, additional parameters defining the extent of the region exposed are:                electron density;        the probability of scattering/impulse transfer;        the angle distribution of the scattered primary electrons;        the angle distribution of the generated secondary electron;        the mean free path of both types of electrons (in principal, a reduction in electron energy causes the mean free path to increase, e.g., to around 5 nm for 5 eV˜248 nm).        
For a more detailed explanation of this theory see: David T. Attwood, Soft x-rays and extreme ultraviolet radiation: principles and applications, Cambridge University Press, 1999 (ISBN 0 521 65214 6), and P. W. H de Jager, An instrument for Fabrication and Analysis of Nanostructures Combining Ion and Electron Regulation, Delft University Press, 1997 (ISBN 90 407 1478 9).
FIG. 1 illustrates schematically the various processes outlined above. As shown, the energy which gives rise to the actual exposure of the resist can result from any one of these processes, and in particular as a result of secondary electron generation. Having regard to a diffraction limited image of a point source, the mean free path of the secondary electrons forms a radius within which photo-chemical effects establish effective exposure of the resist. This radius limits the lowest achievable resolution. The minimum line edge roughness (LER) of any feature is defined, to a first order, by the random generated path of scattered secondary electrons through the resist and the randomized distribution of photons. The LER can be visualized as the envelope of a series of circles (having a radius equal to the random generated path of scattered secondary electrons) centered on a line with the edges of the circles just touching. In addition, the centers of the circles is statistically defined and depends upon the atom density within the resist. This is illustrated in FIG. 2.
As well as adversely affecting the exposure resolution, secondary electrons can also result in damage to layers of an integrated circuit beneath the resist layer. Two scenarios are conceivable:                the voltage of the secondary electrons collected together in the resist layer gives rise to a voltage which leads to damage; and        the secondary electrons travel into a sensitive layer and damage bindings or structures within that layer.        
The collection of electrons within the resist layer may also adversely affect small structures located in the vicinity of large exposed areas or structures. The desired critical dimensions (that is the smallest space between two features of a pattern, such as, for example, lines or contacts, permitted in the fabrication of a device layer and/or the smallest width of a line or any other feature) might also be changed.